Addition curing PMR (polymerization of monomer reactants) of polyimide materials is gaining wide acceptance for matrix resins for high temperature composite structure and engine applications. Considerable cost and weight savings can be achieved by using fiber-reinforced polyimide composites in place of metals currently used in the moderately high temperature zones (up to 600.degree. F.) of aircraft. PMR polyimide materials are traditionally prepared from three monomeric components: an aromatic diamine, a diester-diacid, and an ester-acid endcap, which are mixed in a particular molar ratio to achieve the required molecular weight of the prepolymer. Typical examples of these materials are the monomers used in the production of PMR-15: 4,4'-methylenedianiline (MDA), 3,3',4,4'-benzophenonetetracarboxylic acid methyl ester (BTDE), and 5-norbornene-2,3-dicarboxylic acid methyl ester (NDE) with molar ratio of 3:2:2. In practice, an alcohol solution of these monomers is used in a first step to impregnate reinforcing fibers; most of the solvent is driven off, but it is assumed that no chemical reaction occurs at this stage. In the next stage of the process, the impregnated materials are heated to initiate the condensation reactions that give the amide and imide prepolymers. During the third and final stage of the process, the material is heated under pressure to a temperature adequate to initiate the addition crosslinking reaction of the norbornenyl endcaps.
The conventional PMR-15 approach has several severe limitations: (i) The traditional PMR-15 material is supplied as a prepregging varnish. The alcohol varnish contains three monomers that are reacted after prepregging, resulting in the amic acid which is then imidized to form a nadic end-capped prepolymer at 100.degree.-250.degree. C. In the idealized sequence, these condensation reactions are complete prior to the thermal crosslinking step; however, it has been reported recently in literature that in many processing situations, these three monomers may not completely react as theoretically expected and will yield a complex mixture of reaction products because it is difficult to precisely control the stoichiometry of three different monomeric components due to different levels of impurities and difference in chemical reactivities. Further, there is no easy way to monitor the reaction of the PMR approach. (ii) One of the monomers, MDA, which is normally present at a level of 30 wt% of solid in the prepregging varnish and may not completely react, is now receiving more attention because it is a suspected human carcinogen. Most recently, OSHA proposed new standards for exposure of workers to MDA, including the 8-hour time-weighted average to 10 parts per billion (ppb) and establishing a short-term exposure limit to 100 ppb. (iii) The process of traditional PMR-15 is very dependent on the manufacturing technique as well as on the part thickness. That is why after 15 years since the PMR-15 approach was disclosed, people are still reporting on how to process this resin system. (iv) Reproductibility in both the processing and final properties of this resin have been very poor. (v) A major weakness of traditional PMR-15 polyimides is poor toughness. A novel process which could eliminate these weak points of a traditional PMR approach and could produce better quality materials over PMR materials would be welcome.
This invention provides a process for the production of norbornenyl-capped amide/imide prepolymers, in solution or in particulate form, derived from three reactants: aromatic diamine, dianhydride, and nadic or maleic anhydride. The process comprises several sequential steps so that the reactions of the three monomeric components could be better controlled and monitored and, consequently, the aforementioned weaknesses of PMR approach could be eliminated and the quality-control of the end-capped prepolymer would be improved.
In a recently filed application, Ser. No. 523,349, filed May 14, 1990, polyimide powders are disclosed as being produced from a process whereby an aromatic fluorine-containing diamine is first treated with an end-capping agent and, in a second step, the product of such treatment is reacted with an aromatic dianhydride. After chemically imidizing, the polyimide powder is separated.